Method for forming voids and structure with voids formed using the same

ABSTRACT

A method for forming voids corresponding to pads of SMT components is provided. The method comprises following steps: One or more condition parameters are inputted into a searching unit. The searching unit searches all of the pads with reference to the condition parameters to obtain a pre-selected group of pads. A judgment unit is provided to determine whether each pad of the pre-selected group of pads meets a pre-determined processing requirement to generate a to-be-processed group of pads. An execution unit executes a void formation step with reference to corner coordinates of each of the to-be-processed group of pads, so as to form at least a void at the portion of a contact surface corresponding to a corner of the pad. In an embodiment, four voids which are related to respective corners of each pad of the to-be-processed group are formed at the contact surface accordingly.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of pending U.S. patent application Ser.No. 14/219,042, filed Mar. 19, 2014, and entitled “METHOD FOR FORMINGVOIDS AND STRUCTURE WITH VOIDS FORMED USING THE SAME”. This applicationclaims the benefit of Taiwan application Serial No. 102137165, filedOct. 15, 2013, the subject matter of which is incorporated herein byreference.

BACKGROUND OF THE DISCLOSURE

1. Field of the Disclosure

The disclosure relates in general to a method for forming voids and astructure with voids formed using the same, and more particularly to amethod for forming voids capable of resolving tombstone effect forcomponents and a structure thereof.

2. Description of the Related Art

Along with the miniaturization and sophistication of electric products,such as photoelectric, communication and semiconductor products, thecomponents used in laptops or mobile devices are getting slimmer andlighter. However, the surface mount technology (SMT) used for disposingcomponents will result in more and more defects as the number of smallcomponents increases.

In the design of printed circuit board, if the pad located at one end ofan element contacts a large copper foil while the pad located at theother end does not contact a large copper foil, the temperature of thepads at two ends of the element will be different during the reflowingprocess. FIG. 1A˜FIG. 1D are schematic diagrams of an element tombstoneeffect. As indicated in FIG. 1A, the solders 12 a and 12 b on the padslocated at two ends of the element 11 are not melted and deformed.During reflow, the solders 12 a and 12 b approach reflow temperature,and the solders 12 a and 12 b begin to wet unevenly. The pads 12 areheated at different temperatures, the solder 12 a at a highertemperature will be melted faster than the solder 12 b at a lowertemperature. Also, surface tension and torque increase on one side ofthe solder (such as the solder 12 a). As indicated in FIG. 1B, theelement 11 tilts at one end and causes the melted solder 12 a′ to tumbledown. As indicated in FIG. 1C, during the melting process, the solder 12a′, being affected by unstable temperature in the finance and trackvibration, will flow towards the solder 12 b′ located at the right handside, and the surface tension will make the element 11 rotated andstanding on end during reflow as indicated in FIG. 1D. Such phenomenonis called a ‘tombstone effect’. The lighter the element, the easier topull the element upward and make the element stand on end during reflow.

SUMMARY OF THE DISCLOSURE

The disclosure is directed to a method for forming voids and a structurewith voids formed using the same. Through logic operation, a dataprocessor effectively executes a void formation step on selected pads toform voids at the portion of a contact surface (such as a copper foil)corresponding to respective corners of the pad (such as the four cornersof the pad) to effectively reduce the tombstone effect of components.

According to one embodiment of the present disclosure, a method forforming voids is provided. The method at least comprises:

inputting one or more condition parameters into a searching unit;

searching all of the pads with reference to the one or more conditionparameters by the searching unit to obtain a pre-selected group of pads;

providing a judgment unit to determine whether each pad of thepre-selected group of pads meets a pre-determined processing requirementto generate a to-be-processed group of pads; and

providing an execution unit to execute a void formation step withreference to corner coordinates of each pad of the to-be-processed groupof pads, so as to form at least a void at the portion of a contactsurface corresponding to a corner of the pad.

In an embodiment, four voids which are related to respective corners ofeach pad of the to-be-processed group are formed at the contact surfaceaccordingly.

According to another embodiment of the present disclosure, a structurewith voids is provided. The structure comprises a circuit board and apad. The circuit board at least comprises a copper foil having a padregion. The pad region has at least a void with width X_(V) and lengthY_(V). A perimeter of the void is equal to two times the sum of thewidth X_(V) and the length Y_(V). A pad with width X_(P) and lengthY_(P) is disposed in the pad region for electrically connecting anelement of the pad to the circuit board. The perimeter of the pad isequal to two times the sum of the width X_(P) and the length Y_(P). Thevoid in the pad region corresponds to a corner of the pad. The widthX_(P) and the length Y_(P) of the pad correspond to the width X_(V) andthe length Y_(V) of the void, respectively. The pad has a minimum wirewidth W, and the difference obtained by deducting the perimeter of thevoid from the perimeter of the pad must be greater than or equal to theminimum wire width W.

The above and other aspects of the disclosure will become betterunderstood with regard to the following detailed description of thepreferred but non-limiting embodiment(s). The following description ismade with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A˜FIG. 1D (Prior Art) are schematic diagrams of an elementtombstone effect.

FIG. 2 is a schematic diagram of a structure with voids according to anembodiment of the disclosure.

FIG. 3 is a flowchart of a method for forming voids of the disclosure.

FIG. 4 is a flowchart of a method for forming voids according to anembodiment of the disclosure.

FIG. 5 is a schematic diagram of contents of the setting of conditionparameter according to an embodiment of the disclosure.

FIG. 6 is a schematic diagram of position and corner coordinates of apad.

FIG. 7A is a flowchart of a method for calculating void width accordingto an embodiment of the disclosure.

FIG. 7B is a schematic diagram of a pad and its corresponding pad regionand voids according to an embodiment of the disclosure.

FIG. 8 is a flowchart of a method for forming voids according to a firstapplication example of the disclosure.

FIG. 9 is a flowchart of a method for forming voids according to asecond application example of the disclosure.

DETAILED DESCRIPTION OF THE DISCLOSURE

A method for forming voids and a structure with voids formed using thesame are provided in embodiments of the disclosure. By forming at leasta void at a portion of a contact surface (such as a copper foil)corresponding to a corner of a pad (such as four corners of the pad),the contact area between the pad and the contact surface (such as thecopper foil) is sufficient, characteristics of elements are maintained,the dissipation rate is increased, the probability of surface tension onmelted pad being decreased is lowered, and the tombstone effect of thecomponents is greatly reduced.

FIG. 2 is a schematic diagram of a structure with voids according to anembodiment of the disclosure. A pad 22, electrically connecting elementand a circuit board, is disposed in a pad region 231 on a copper foil23. Conventionally, the entire pad is conducted with a copper foil 23,that is, a bottom surface of the pad completely contacts the copper foil23. In the present embodiment, at least a void 235 corresponding to acorner of the pad 22 is formed in the pad region 231. As indicated inFIG. 2, the pad region 231 has four voids 235 corresponding to fourcorners of the pad 22, respectively. When the pad 22 is disposed in thepad region 231, the formation of voids 235 helps to increase thedissipation rate and avoid the tombstone effect.

The circuit board has many components disposed thereon and thecomponents may have different sizes. Small components, such asresistor/inductor/capacitor (R/L/C) with package sizes: 0201/0402/0603,are susceptible to the tombstone effect. In general, the size of packagecomponent has nothing to do with resistance but is closely related withpower. For instance, the watts corresponding to package sizes 0201,0402, 0603, 0805, and 1206 are 1/20W, 1/16W, 1/10W, ⅛W, and ¼W,respectively. The corresponding relationship between package size andappearance size of capacitor/resistor is: 0402=1.0 mm×0.5 mm, 0603=1.6mm×0.8 mm, 0805=2.0 mm×1.2 mm, 1206=3.2 mm×1.6 mm, 1210=3.2 mm×2.5 mm,1812=4.5 mm×3.2 mm, 2225=5.6 mm×6.5 mm. Component package refers to theindicated appearance of components soldered to the circuit board andpositions of soldered dots. Different elements may share the samecomponent package, and the same elements may have different componentpackages.

In general, 70% of the components on the printed circuit board areresistor/inductor/capacitor (R/L/C) with package sizes: 0201/0402/0603.It would be too time-consuming to hollow the portion by which a copperfoil contacting the four corners of a pad for every component.Furthermore, the sizes and positions of the hollowed portions areirregular. Therefore, the disclosure provides a method for forming voidson a printed circuit board by a data processor to effectively avoid thetombstone effect.

Referring to FIG. 3, a flowchart of a method for forming voids of thedisclosure is shown. In step 301, one or more condition parameters areinputted into a searching unit (such as a logic unit of a dataprocessor). In step 302, the search unit searches all of the pads withreference to the inputted condition parameters to obtain a pre-selectedgroup of pads. In step 303, a judgment unit is provided to determinewhether each pad of the pre-selected group of pads meets apre-determined processing requirement to generate a to-be-processedgroup of pads. In step 304, an execution unit is provided to execute avoid formation step with reference to corner coordinates of each pad ofthe to-be-processed group of pads, so as to form at least a void (suchas void 235 of FIG. 2) at the portion of a contact surface correspondingto a corner of the pad.

Detailed descriptions related to the embodiments of the disclosure aredisclosed below with reference to accompanying drawings. It should benoted that detailed steps and calculation disclosed in embodiments ofthe disclosure are exemplary and explanatory only and are notrestrictive of the disclosed embodiments as claimed. Therefore, thedescriptions and accompanying drawings of the disclosed embodiments arenot for limiting the scope of protection of the disclosure.

FIG. 4 is a flowchart of a method for forming voids according to anembodiment of the disclosure. In step 401, one or more conditionparameters are inputted into a searching unit (such as a logic unit of adata processor). In an embodiment, the condition parameters, exemplarilybut not restrictively, comprise: a condition parameter of selection bylayer, a condition parameter of particular combination of elementgroups, a condition parameter of selection custom elements, and acondition parameter of setting a minimum void area.

Referring to FIG. 5, a schematic diagram of contents of the setting ofcondition parameter according to an embodiment of the disclosure isshown. Condition parameters are inputted through a program interface toadjust the range of operation, save the running time, and set correctexecution targets. The pads requiring void formation can be selectedwith reference to ticked condition parameters. The condition parametersof FIG. 5 are described below.

By Layer: If this item is selected, void formation will be executed onthe full layer. If this item is not selected, then some particular oruser-defined regions will be selected instead. In comparison to theselection by layer, selection by region involves a smaller range of voidformation, hence saving more running time.

Include All *201*, *402*, and *603* packages: If this item is selected,then the step of forming void corresponding to a corner of the pad willcover all components with package name *201*, *402*, or *603*. This itembelongs to the condition parameter of selection by particularcombination of element groups. For instance, if the item ‘Include All*201*, *402*, and *603* packages’ is selected, then all components withkey word *805* in the region will be excluded from the processing list.In practical application, the content of the combination can be modifiedaccording to actual requirements and conditions, other combination ofparticular element groups can be added, and the package names are notlimited to *201*, *402*, and *603* exemplified in FIG. 5.

Include From File: If this item is selected, which components must beselected can be defined in a text file. This item belongs to thecondition parameter of selection by custom elements, and requiredpackage names can be inputted according to the needs in practicalapplication. The present condition parameter can be selected togetherwith the previous condition parameter of selection by a particularcombination of element groups.

Min void area to be deleted [sq mil]: If this item is selected, then aminimum void area is set. Due to the limitation in the manufacturingability of PCB manufacturers, if the minimum area of a void region issmaller than the minimum value, then the PCB manufacturers will notformat the void 235. With the minimum void area being set, when the voidarea of the pad obtained in subsequent step is smaller than the setvalue, void formation is not executed.

In an embodiment, the condition parameters inputted through programinterface may comprise the conditions of selecting components fromdifferent layers (such as an upper layer or a lower layer), and settinga minimum interval between the voids 235 of the pads of adjoiningelements, and not executing void formation if the interval is smallerthan the minimum interval. In practical application, the contents ofcondition parameters can be modified with reference to the conditions inactual application such that the provided program interface can meetactual needs and is not limited to the exemplification of FIG. 5.

In step 402, the searching unit searches all of the pads with referenceto the inputted condition parameters to obtain a pre-selected group ofpads. In an embodiment, the searching unit can generate a pre-selectedlist.

In step 403, the judgment unit determines whether each pad of thepre-selected group of pads meets a pre-determined processingrequirement. If the judgment unit determines that a pad meets thepre-determined processing requirement, then the method proceeds to step404. In step 404, the pad is categorized into a to-be-processed group ofpads by the judgment unit. If the judgment unit determines that a paddoes not meet the pre-determined processing requirement, then the methodproceeds to step 405. In step 405, the pad is categorized into ahalt-processing group of pads by the judgment unit.

In an embodiment, the pre-determined processing requirement for a pad isthat the position coordinates of the pad of pre-selected group of padsare located in a copper region. The pads whose position coordinates arelocated in the copper region are determined by the judgment unit asmeeting the pre-determined processing requirement, and are categorizedinto to-be-processed group of pads (step 404). The pads whose positioncoordinates are not located in the copper region do not need to beprocessed further and are categorized into a halt-processing group ofpads (step 405).

In an embodiment, the judgment unit generates a halt-processing listafter generating a halt-processing group of pads, and thehalt-processing list can be combined and outputted together with areport file (step 408).

In step 406, the execution unit executes a void formation step withreference to the corner coordinates of at least a corner (such as fourcorners) of each pad of the to-be-processed group of pads, so as to format least a void at the portion of a contact surface corresponding to acorner of the pad (such as void 235 of FIG. 2). In an embodiment, thepad is disposed on a copper foil, and the execution unit executes a voidformation step on the portion of the copper foil contacted by the pad(corresponding to four corners of the pad), and the voids are formed onthe copper foil.

In an embodiment, after the execution unit executes a void formationstep, the judgment unit generates a void formation processed list (step407), and outputs a report file which comprises a void formationprocessed list and a halt-processing list (step 408). The program pops areport file, and the method terminates.

<Calculation of Corner Coordinates of a Pad>

In an embodiment, the searching unit has a database storing packagenames of all elements, and the sizes and position coordinates of thepads corresponding to the elements. FIG. 6 is a schematic diagram ofposition and corner coordinates of a pad. The position coordinates ofthe pad stored in the database is such as the center coordinates (x, y).It is assumed that the pad 62 has a length Y_(P) and a width X_(P), thenthe coordinates of corners A, B, C and D of the pad 62 are: (x−X_(P)/2,y+Y_(P)/2), (x+X_(P)/2, y+Y_(P)/2), (x+X_(P)/2, y−Y_(P)/2) and(x−X_(P)/2, y−Y_(P)/2), respectively. The four voids 635 in the padregion correspond to the four corners A, B, C and D of the pad 62.

In an embodiment, the corner coordinates of the pad 62 can be calculatedand inputted to the database before the selection of pads. In anotherembodiment, the corner coordinates of the pad 62 can be obtained withreference to the pads requiring void formation after the selection ofpads (the judgment unit generates a to-be-processed group of pads).Suitable programs can be designed according to actual application, butthe disclosure is not limited thereto.

Referring to FIG. 6. Suppose the pad has a length Y_(P) and a widthX_(P). Then, the perimeter of the pad is equal to two times the sum ofthe width X_(P) and the length Y_(P). That is, the perimeter of thepad=(Y_(P)+X_(P))×2. Suppose the void 635 in the pad region has a lengthY_(V) and a width X_(V). Then, the perimeter of the void is equal to twotimes the sum of the width X_(V) and the length Y_(V). That is, theperimeter of the void=(Y_(V)+X_(V))×2. The width X_(P) and the lengthY_(P) of the pad 62 correspond to the width X_(V) and the length Y_(V)of the void 635 respectively. In an embodiment, exemplarily but notrestrictively, the four voids are four squares of the same size.

<Calculation of Void Width ΔX>

In an embodiment, the judgment unit can obtain a void width ΔXcorresponding to each corner of a pad according to a required wire widthand a pad size of each element. In an embodiment, the required wirewidth of each element can be obtained with reference to a required loadcurrent of each element and a thickness of a copper foil (a contactsurface) at which the pad is located.

Referring to FIGS. 7A and 7B. FIG. 7A is a flowchart of a method forcalculating void width according to an embodiment of the disclosure.FIG. 7B is a schematic diagram of a pad and its corresponding pad regionand voids according to an embodiment of the disclosure. In anembodiment, the four corners of the pad can be arc-shaped orrectangular. The corners illustrated in FIG. 7B are arc-shaped, but thedisclosure is not limited thereto. As indicated in FIG. 7B, the pad isdisposed in the pad region 731 of the copper foil 73, and the fourcorners of the pad respectively correspond to the center of the voids735.

Referring to FIG. 7A. In step 701, a required load current of eachelement is determined with reference to element specifications (such asby looking up relevant tables). In step 702, a required wire width ofeach element is determined with reference to a required load current ofeach element and a thickness of a copper foil at which the pad islocated. In example 1, suppose the result of table search confirms thata particular element carries 2 A current. If the thickness of the copperfoil at which the element is located is 35 μm., then the required wirewidth of the element can be obtained from formula (1-1). The calculationresult shows that the required wire width is about 0.8 mm=31 mil.

Based on the design standards of printed circuit board such as IPC-2221or IPC-4101, the required wire width of a component can be determinedwith reference to condition parameters such as copper foil thickness andmaximum current.

W=(1/1.4h)*[I/(k*ΔT ^(0.421))]^(1.379)  (1-1)

W: minimum wire width;I: maximum current allowed;ΔT: maximum temperature rise;h: copper thickness (oz/μm²);(h=25 μm if the copper wire is disposed in an inner layer and h=46 μm ifthe copper wire is disposed in an outer layer);k: dielectric constant;(k=0.024 if the copper wire is disposed in an inner layer and h=0.048 ifthe copper wire is disposed in an outer layer).

In step 703, a void width ΔX corresponding to the corner of each pad iscalculated with reference to the required wire width and a pad size ofeach element. Referring to FIG. 6. The minimum wire width W iscalculated according to formula (1-1), and the width of FIG. 6 being[(Y_(P)+X_(P))×2−(Y_(V)+X_(V))×2] must be greater than or equal to W.Suppose Y_(P)=X_(P) and Y_(V)=X_(V). Then, Y_(P)−Y_(V)≧W/4 orX_(P)−X_(V)≧W/4. Referring to FIG. 7B. As disclosed in example 1, if oneelement requires a minimum wire width being 0.8 mm=31 mil, then thewidth of FIG. 7B being (Y2+X2)×2 must be greater than or equal to theminimum wire width being 31 mil. Suppose Y2=X2. Then, Y2=X2=31÷4≈8, andthe void width ΔX≦(X1−8), wherein X1 is determined with reference to thepad size of the element (such as components 0201/0402/0603, and so on).

In an embodiment, in addition to the package names of all elements, andthe sizes and position coordinates of the pads corresponding to theelements, the database further stores a void width ΔX corresponding tothe corner of each pad. After the judgment unit generates ato-be-processed group of pads, the judgment unit can refer to the voidwidth ΔX stored in the database, and controls the execution unit to formvoids at the portion of a copper foil corresponding to the corner ofeach pad of the to-be-processed group of pads. The present embodiment isexemplified by a first application example below.

FIG. 8 is a flowchart of a method for forming voids according to a firstapplication example of the disclosure. In step 801, one or morecondition parameters are inputted into a searching unit. The searchingunit has a database storing package names of all elements, and the sizesand position coordinates of the pads corresponding to the elements,corner coordinates corresponding to the corner of each pad and a voidwidth ΔX (obtained through calculation). In step 802, the searching unitsearches all of the pads with reference to the inputted conditionparameters and the database to obtain a pre-selected group of pads. Instep 803, the judgment unit determines whether each pad of thepre-selected group of pads is located in the copper region (that is,meets a pre-determined processing requirement). If the pad is located inthe copper region, then the method proceeds to step 804. In step 804,the pad is categorized into a to-be-processed group of pads pad by thejudgment unit to generate a to-be-processed list. If the pad is notlocated in the copper region, then the method proceeds to step 805. Instep 805, the pad is categorized into a halt-processing group of pads bythe judgment unit which further generates a halt-processing list. Instep 806, the execution unit forms a void at the portion of the contactsurface corresponding to a corner of the pad with reference to the voidwidth ΔX and the corner coordinates of each pad of the to-be-processedgroup of pads (obtained from the inputted values of the database). Afterthe execution unit finishes the void formation step, the method proceedsto step 807. In step 807, the judgment unit generates a void formationprocessed list. In step 808, the judgment unit outputs a report filecomprising a void formation processed list and a halt-processing list.The program pops a report file, and the method terminates.

In another embodiment, after generating a to-be-processed group of pads,the judgment unit may calculate corner coordinates of each pad of theto-be-processed group of pads and a void width ΔX corresponding to thecorner with reference to the size and position coordinates of each padstored in the database. Then, the execution unit obtains the void widthΔX and the corner coordinates of the pad through calculation so as toform a void at the portion of the contact surface corresponding to acorner of the pad. The present embodiment is exemplified by a secondapplication example below.

FIG. 9 is a flowchart of a method for forming voids according to asecond application example of the disclosure. In step 901, one or morecondition parameters are inputted into a searching unit which has adatabase storing package names of all elements, and the sizes andposition coordinates of the pads corresponding to the elements. In step902, the searching unit searches all of the pads with reference to theinputted condition parameters and database to obtain a pre-selectedgroup of pads. In step 903, the judgment unit determines whether eachpad of the pre-selected group of pads is located in the copper region(that is, meets a pre-determined processing requirement). If the pad islocated in the copper region, then the method proceeds to step 904. Instep 904, the pad is categorized into a to-be-processed group of pads.If the pad is not located in the copper region, then the method proceedsto step 905. In step 905, the pad is categorized into a halt-processinggroup of pads. In step 906, the judgment unit calculates the cornercoordinates of each pad of the to-be-processed group of pads and thevoid width ΔX corresponding to a corner of the pad. In step 907, theexecution unit forms a void at the portion of the contact surfacecorresponding to a corner of the pad with reference to the cornercoordinates of each pad of the to-be-processed group of pads and thevoid width ΔX. In step 908, after the execution unit finishes a voidformation step, the judgment unit generates a void formation processedlist. In step 909, the judgment unit outputs a report file comprising avoid formation processed list and a halt-processing list. The programpops a report file, and the method terminates.

According to the method for forming voids and the structure with voidsformed using the same disclosed in above embodiments, a void is formedat the portion of a contact surface (such as a copper foil)corresponding to a corner of the pad (such as the four corners of thepad), such that the contact area between the pad and the contact surface(such as the copper foil) is sufficient, characteristics of elements aremaintained, and the tombstone effect of components can be reduced. In anembodiment, a data processor, which executes logic processing andoperation, generates a to-be-processed group of pads with reference tothe inputted conditions and executes a void formation step on theselected pads, such that the processing time can be reduced, and thesizes and positions of the pads are formed in a regular manner. Througha program interface with suitable design, the void formation can be moreflexibly selected and the inputted conditions can be modified to meetthe requirements of different applications. Therefore, the voidformation can be executed more effectively and flexibly, the tombstoneeffect of components can be reduced, and product conformity rate can beincreased.

While the disclosure has been described by way of example and in termsof the preferred embodiment(s), it is to be understood that thedisclosure is not limited thereto. On the contrary, it is intended tocover various modifications and similar arrangements and procedures, andthe scope of the appended claims therefore should be accorded thebroadest interpretation so as to encompass all such modifications andsimilar arrangements and procedures.

What is claimed is:
 1. A structure with voids, comprising: a circuitboard, comprising at least a copper foil having a pad region, whereinthe pad region has at least a void with width X_(V) and length Y_(V),and a perimeter of the void is equal to two times a sum of the widthX_(V) and the length Y_(V); a pad with width X_(P) and length Y_(P),wherein the pad is disposed in the pad region for electricallyconnecting an element of the pad to the circuit board, and has aperimeter equal to two times a sum of the width X_(P) and the lengthY_(P), the void of the pad region corresponds to a corner of the pad,the width X_(P) and the length Y_(P) of the pad correspond to the widthX_(V) and the length Y_(V) of the void, respectively, wherein, the padhas a minimum wire width W, and the difference obtained by deducting theperimeter of the void from the perimeter of the pad must be greater thanor equal to the minimum wire width W.
 2. The structure according toclaim 1, wherein the pad region has the four voids corresponding to thefour corners of the pad respectively.
 3. The structure according toclaim 2, wherein the voids have identical sizes.
 4. The structureaccording to claim 2, wherein the voids are square in shape.